Power system is a network of electrical components used to supply, transmit and use electric power. Transmission lines are considered as main elements in the power system. Any disturbances occurring in the transmission lines need to be detected, and isolated as soon as they occur if they indicate a fault. Therefore high speed and secured protection is essential to maintain a reliable power system to satisfy customer daily needs. The transmission lines are protected using impedance relays usually arranged in a substations of the power system. Patent WO 2013/064176 A1 discloses one of such examples. According to the patent, a method of fault detection in a power transmission line is provided. The fault detection is based on fault-induced travelling waves. In particular, a voltage sensor and a current sensor such as voltage transformer and current transformer, are arranged for measuring the voltage and current at an end of the transmission line, and an estimating means is arranged for estimating a pre-fault voltage and current as well as the amplitude of the fault-induced travelling wave based on the voltage and current measurements performed by the voltage sensor and current sensor. In the substation, the secondary sides of current transformer and voltage transformer are directly connected to the relay by cable wires.
With the development of digital technology, a power system within a digital substation becomes more and more popular, where a merging unit is arranged between the electrical parameter sensor (such as voltage sensor and current sensor) and the intelligent electronic device (IED). Electrical parameters measured by the electrical parameter sensors at one end of the transmission line are communicated to IED successively via the merging unit and a process bus. Regards travelling wave protection, the electrical parameter sensor normally needs higher sampling rate to acquire the high frequency transient of travelling wave front, for example 10 kHz to 10 MHz. While according to the IEC61850 standard for a digital substation, the sampling rates of merging unit for protection is 4.8 k Hz for 60 Hz power systems and 4 kHz for 50 Hz power systems. The process bus schemes for 60 Hz power systems and 50 Hz power systems are also respectively designed for the sampling rates at 4.8 k Hz and 4 kHz as well. The bandwidth of the process bus is not broad enough for transmitting the samples from the higher sampling rate of the electrical parameter sensor. Thus, the bandwidth limitation of the process bus can be considered as a main challenge or bottleneck to implement travelling wave based protection and fault location used for the power system within a digital substation environment.